Dye sensitization photoelectric converter and process for producing the same

ABSTRACT

A dye-sensitized photoelectric conversion apparatus having enhanced energy conversion efficiency and a production method thereof are provided.  
     The dye-sensitized photoelectric conversion apparatus which has semiconductor layer ( 13 ) containing a photosensitizing dye ( 14 ) and is constituted such that a charge carrier generated by allowing light to incident in the photosensitizing dye ( 14 ) is drawn out through the semiconductor layer ( 13 ), in which the semiconductor layer ( 13 ) is constituted by a plurality of regions ( 13 A to  13 D) having different energy levels from one another of a passage through which the charge carrier is transferred. Further, the plurality of regions ( 13 A to  13 D) are arranged such that the energy levels are reduced stepwise and/or continuously in the direction of drawing the charge carrier out.

TECHNICAL FIELD

The present invention relates to a dye-sensitized photoelectricconversion apparatus applicable to a solar cell and the like and amanufacturing method thereof.

BACKGROUND ART

A solar cell utilizing sunlight is receiving attention as an energysource that can substitute for a fossil fuel, and various types ofstudies have been exerted. The solar cell is one type of photoelectricconversion apparatus which can convert light energy into electricenergy.

The solar cell which utilizes a pn junction of a semiconductor is mostwidely distributed, but, since it is necessary to conduct a step ofmanufacturing a semiconductor material of high purity or a step offorming the pn junction, there is a problem in that an economical costand an energy cost in a manufacturing process are high.

On the other hand, a dye-sensitized photochemical cell which utilizes alight-excited electron transfer has been proposed by Graetzel et al.(Japanese Patent No. 2664194; J. Am. Chem. Soc. (1993), 115, 6382 to6390; Nature (1991), 353, 737; and the like) and is expected to be thesolar cell of the new generation capable of being produced at low costby using a low-priced material.

FIG. 6 is a schematic cross-sectional diagram showing an example of aconventional representative dye-sensitized photochemical cell. Thedye-sensitized photochemical cell is primarily constituted by atransparent substrate 1 such as glass, a transparent electrode (negativeelectrode) 2 comprising a transparent conductive film of, for example,ITO (Indium Tin Oxide), a semiconductor layer 3, a photosensitizing dye4 adsorbed on a surface of the semiconductor layer 3, a counterelectrode (positive electrode) 6, an electrolyte layer 5 sandwichedbetween the semiconductor layer 3 and the counter electrode 6, anothersubstrate 7, a sealing material 8 and the like.

As for the semiconductor layer 3, a porous article prepared by sinteringfine grains of titanium oxide TiO₂ is used in many cases. On a surface,on the side of the electrolyte layer 5, of the semiconductor 3, aphotosensitizing dye 4 is adsorbed. As for the photosensitizing dye 4, amaterial having an absorption spectrum in the vicinity of a visiblelight region such as a ruthenium complex is used. As for the electrolytelayer 5, an electrolyte solution containing an oxidation-reductionsystem (redox pair) such as I⁻/I₂ (however, actually, I₂ is combinedwith I⁻ and exists in a form of I₃ ⁻.) is mentioned.

An apparatus as shown in FIG. 6 operates as a cell in which the counterelectrode 6 is a positive electrode and the transparent electrode 2 is anegative electrode, when light is incident in the apparatus. A theory ofsuch operation is as described below.

When the photosensitizing dye 4 absorbs a photon which passes throughthe semiconductor layer 3, an electron inside the photosensitizing dye 4is excited to undergo transition from a ground state to an excitedstate. The electron in the excited state is quickly drawn out into aconduction band of the semiconductor layer 3 via an electric bondbetween the photosensitizing dye 4 and the semiconductor layer 3, passesthrough the semiconductor layer 3 and, then, reaches the transparentelectrode 2.

On the other hand, the photosensitizing dye 4 which has been oxidized bylosing the electron receives an electron from a reducing agent (forexample I⁻) in the electrolyte layer 5 to be reduced. The reducing agent(for example, I₂) which has lost the electron reaches the counterelectrode 6 by a diffusion effect and, there, receives an electron fromthe counter electrode 6 and is, accordingly, reduced and is back to anoriginal reducing agent.

In such a manner as described above, light energy is converted intoelectric energy without leaving any change in each of thephotosensitizing dye 4 and the electrolyte layer 5.

Most important points for effectively operating a photoelectricconversion element are: efficiently absorbing light; efficientlygenerating-separating a charge carrier (for example, electron) from theexcited state caused by absorbing the light energy; and quickly drawingthe thus-separated charge carrier outside as a current.

In the dye-sensitized photochemical cell, light absorption is borne bythe photosensitizing dye 4 and efficient absorption can be attained byselecting an optimum photosensitizing dye 4.

Generation and separation of the charge carrier from the excited stateis performed at an interface between the photosensitizing dye 4 and thesemiconductor layer 3. Namely, while the electron is drawn from thephotosensitizing dye 4 in the excited state into the conduction band ofthe semiconductor layer 3, the photosensitizing dye 4 which has lost theelectron stays on the surface of the semiconductor layer 3, to therebyattain the generation and separation of the charge carrier.

However, since a subsequent movement of the electron in thesemiconductor layer 3 is relied on a diffusive migration, some electronswhich are each combined with a hole in the semiconductor layer 3 orrecombined with the photosensitizing dye which has lost the electron atthe interface between the semiconductor layer 3 and the photosensitizingdye 4 and, accordingly, can not reach the transparent electrode 2 aregenerated. Since these electrons can not be drawn out as an electriccurrent, the electrons cause a reduction of energy conversionefficiency.

In an effort to enhance the energy conversion efficiency of thedye-sensitized photochemical cell, studies and developments are inprogress in various fields. As far as the semiconductor layer isconcerned, besides titanium oxide TiO₂, not only oxide semiconductorsand the like such as Nb2O5 and ZnO are used, but also a complex formthereof, namely, an electrode made of a tin oxide grain zinc•oxide grainmixture, or another electrode made of a complex in which a tin oxidegrain is subjected to a surface treatment by a heterogeneous metal oxideis used. (Newest Technology of Dye-Sensitized Solar Cell, supervised andedited by Hironori Arakawa, Chapters 16 and 17, CMC (2001)). However, aconcrete guide line for forming a complex while taking an energy levelof the semiconductor into consideration has not yet been established.

Under these circumstances, the present invention has been attained andan object thereof is to provide a dye-sensitized photoelectricconversion apparatus in which an energy conversion efficiency has beenenhanced and a manufacturing method thereof.

DISCLOSURE OF THE INVENTION

Namely, the present invention relates to a dye-sensitized photoelectricconversion apparatus, comprising a semiconductor layer comprising aphotosensitizing dye, being constituted such that a charge carriergenerated by allowing light to be incident in the photosensitizing dyecan be drawn out through the semiconductor layer, wherein thesemiconductor layer is formed by a plurality of regions having differentenergy levels from one another of a passage through which the chargecarrier is transferred and comprises the regions in which the energylevels of the semiconductor layer are reduced stepwise and/orcontinuously in the direction of drawing the charge carrier out.

Further, the invention relates to a method for producing adye-sensitized photoelectric conversion apparatus which comprises asemiconductor layer comprising a photosensitizing dye and is constitutedsuch that a charge carrier generated by allowing light to be incident inthe photosensitizing dye can be drawn out through the semiconductorlayer, comprising the steps of: constituting the semiconductor layer bya plurality of regions having different energy levels from one anotherof a passage through which the charge carrier is transferred; andarranging the plurality of regions such that the energy levels arereduced stepwise and/or continuously in the direction of drawing thecharge carrier out.

According to the invention, since the region in which the energy levelof the passage through which the charge carrier is transferred isreduced in the direction of drawing the charge carrier out is formed inat least one portion of an inside part of the semiconductor layer whichis the passage of the charge carrier leading from the interface betweenthe semiconductor layer and the photosensitized dye to the electrode fordrawing the charge carrier out, the charge carrier is subjected to aforce in the direction of drawing the charge carrier out in the regionand, accordingly, a movement of the charge carrier is set to be in thedirection of drawing the charge carrier out.

Namely, since the transfer of the charge carrier after being injected inthe semiconductor layer is controlled based on the energy level of thepassage, through which the charge carrier is transferred, in thesemiconductor layer, in comparison with a case in which the transfer ofthe charge carrier is relied on a diffusive migration, the number of thecharge carriers which can reach the electrode from which the chargecarriers are drawn out is increased, to thereby enhance the energyconversion efficiency.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B are a schematic cross-sectional diagram of anillustrative example of a dye-sensitized photochemical cell according toEmbodiment 1 and an enlarged cross-sectional view of a major portionthereof, respectively.

FIG. 2 illustrates LUMO and HOMO of various types of semiconductormaterials in the order of from a semiconductor material having a highLUMO to that having a low LUMO.

FIG. 3 is an enlarged cross-sectional view of a major portion of adye-sensitized photochemical cell according to Embodiment 2.

FIGS. 4A to 4G are a schematic cross-sectional diagram explaining amanufacturing process of a semiconductor layer comprising a complex of atitanium oxide thin film and a titanium oxide fine grain having a driftregion of an electron inside according to Embodiment 3 and a schematiccross-sectional diagram explaining a manufacturing process of asemiconductor layer comprising a complex of a titanium oxide thin filmand a titanium oxide fine grain having a drift region of an electroninside.

FIGS. 5A and 5B are a schematic cross-sectional diagram explaining amanufacturing process of a semiconductor layer comprising a complex of atitanium oxide thin film and a titanium oxide fine grain having a driftregion of an electron inside according to Embodiment 4 and an enlargedcross-sectional view of a major portion thereof.

FIG. 6 is a schematic cross-sectional diagram of an illustrative exampleof a conventional dye-sensitized photochemical cell.

BEST MODE FOR CARRYING OUT THE INVENTION

According to the present invention, preferably, the semiconductor layeris formed by a plurality of layers having different minimum energylevels from one another of conduction band and the energy levels arereduced stepwise and/or continuously in the direction of drawing thecharge carrier out.

Preferably, the plurality of regions are constituted by a plurality ofsemiconductor materials in which constitutional elements are differentfrom one another, or semiconductor materials comprising sameconstitutional elements with one another such that ratios of theconstitutional elements are changed stepwise and/or continuously in thedirection of drawing the charge carrier out.

Further, preferably, the plurality of regions are constituted bysemiconductor materials comprising a same element composition anddifferent dopants from one another or are constituted by materials inwhich a same dopant is doped in a semiconductor material having a sameelement composition such that a concentration of the dopant is changedstepwise and/or continuously in the direction of drawing the chargecarrier out.

Further, preferably, on the side to which the photosensitizing dye isadhered, an irregular contour is formed on a first semiconductor layerand, by joining the irregular contour with a second semiconductor layer,the semiconductor layer is constituted.

According to the invention, preferably, the semiconductor layercomprising the photosensitizing dye and an electrolyte layer arelaminated one on the other between a pair of electrodes, to therebyconstitute a dye-sensitized photochemical cell.

According to the invention, preferably, the semiconductor layer isconstituted by a plurality of layers having different minimum energylevels from one another of conduction band and the energy levels arereduced stepwise and/or continuously in the direction of drawing thecharge carrier out.

Preferably, the plurality of regions are formed by laminating one on theother a plurality of semiconductor materials having differentconstitutional elements from one another one on the other or laminatinga plurality of semiconductor materials, having a same constitutionalelement, in which ratios of the constitutional elements are changedstepwise and/or continuously in the direction of drawing the chargecarrier out.

On this occasion, as for a laminating method, preferably, a step offorming a thin film comprising the semiconductor material by asputtering method or a sol-gel method, or a step comprising coating of adispersion containing a superfine grain of a semiconductor material,evaporating of a dispersion medium, and sintering, melt-fusing orbonding of the superfine grain is repeatedly performed on the pluralityof semiconductor materials.

Further, preferably, when a step of doping a plurality of types ofdopants in the semiconductor layer by means of an ion implantation isperformed, the plurality of regions are formed by doping a dopant havinga larger effect in reducing the energy level in an inner portion byusing a larger acceleration voltage by means of the ion implantation.

Further, preferably, when a step of doping a single type of dopant inthe semiconductor layer by means of an ion implantation is performed,the ion implantation by a large acceleration voltage is performed with ahigh dosage while the ion implantation by a small acceleration voltageis performed with a low dosage. Thus, the plurality of regions areformed.

Further, preferably, the energy level is changed by implanting an oxygenion in the semiconductor layer.

Preferably, when a step of doping a plurality of types of dopants byintroducing a dopant-containing gas into an atmospheric gas while thesemiconductor layer is being formed by a sputtering method is performed,the plurality of regions are formed by performing such doping in theorder of from a dopant having a large effect in reducing the energylevel to a dopant having a small effect in reducing the energy level.

Preferably, when a step of doping a single type of dopant by introducinga dopant-containing gas into an atmospheric gas while the semiconductorlayer is being formed by a sputtering method is performed, the pluralityof regions are formed by reducing a concentration of thedopant-containing gas.

According to the invention, preferably, the photosensitizing dye isadhered on a surface of the semiconductor layer or impregnated insidethe semiconductor layer.

When the photosensitizing dye is impregnated inside the semiconductorlayer, after the plurality of regions are formed, there is a case inwhich the photosensitizing dye is impregnated in the plurality ofregions at one time or another case in which the photosensitizing dye isimpregnated to each region while the plurality regions are being formed.

Hereinafter, embodiments according to the invention will be describedwith reference to drawings.

EMBODIMENT 1 A Case in Which Semiconductor Layers Having Different BandStructures From One Another are Laminated

FIGS. 1A and 1B are a schematic cross-sectional diagram of anillustrative example of a dye-sensitized photochemical cell according toEmbodiment 1 and an enlarged cross-sectional view of a major portionthereof taken along a broken line in the schematic cross-sectionaldiagram, respectively.

A transparent substrate 11 is a material having such a property and ashape as allow light to be easily transmitted; for example, a glasssheet or a transparent plastic sheet made of, for example, polyethyleneterephthalate or polycarbonate is used. Since it is not necessary thatthe other substrate 17 is transmittable to light, an opaque glass sheet,a plastic sheet, ceramic sheet or metal sheet may be used.

On a surface of the transparent substrate 11, a transparent electrode 12is formed as an electrode (negative electrode) for drawing an electronout. A material of the transparent electrode 12 is tin oxide doped withantimony or fluorine, indium oxide doped with tin or the like. Thetransparent electrode 12 is formed by any one of a sputtering method, aCVD (Chemical Vapor Deposition) method, a sol-gel method, a vacuumdeposition method, a coating method and the like.

A semiconductor layer 13 is constituted by a plurality of semiconductorlayers having different constitutional elements from one another. Take acase of FIG. 1B as an example, the semiconductor layer 13 is constitutedby semiconductor thin films 13A to 13D comprising 4 types ofsemiconductor materials A to D. Thickness of each layer is in the rangeof from approximately 10 nm to approximately 10 μm.

Materials applicable to constitutional materials of the semiconductorthin films 13A to 13D described in the order of from such materialhaving a high minimum energy level of the conduction band to that havinga low minimum energy level (the former number in each parenthesisdenotes a minimum energy level (LUMO) of the conduction band while thelatter number therein denotes a maximum energy level (HOMO) of valenceelectron band in voltage (V) against reference hydrogen electrodevoltage) are as follows:

GaP (−1, 1.2), ZrO₂ (−1, 4), Si (−0.8, 0.2), CdS (−0.5, 2), KTaO₃ (−0.4,3), CdSe (−0.2, 1.5), SrTiO₃ (−0.2, 3), TiO₂ (0, 2.95), Nb₂O₅ (0, 3.25),ZnO (0, 3.05), Fe₂O₃ (0.2, 2.4), WO₃ (0.3, 2.8), SnO₂ (0.5, 4) and In₂O₃(0.5, 3).

FIG. 2 illustrates LUMO and HOMO of the aforementioned semiconductormaterials.

A plurality of semiconductor materials, that is, four types ofsemiconductor materials A to D in a case of FIG. 1A and 1B, are selectedfrom among the aforementioned semiconductor materials and, then, thinfilms of those materials are laminated on the transparent electrode 12in the order of from the thin film having a low LUMO to that having ahigh LUMO by using the sputtering method, the sol-gel method or thelike.

In such a manner as described above, by laminating a plurality ofsemiconductor materials having different band structures from oneanother on the transparent electrode (negative electrode) 12 in theorder of from the semiconductor material having a low LUMO to thathaving a high LUMO, a structure of an entire semiconductor layer 13 inwhich the energy level of the passage through which a conduction bandelectron is transferred is decreased in the direction of the transparentelectrode (negative electrode) 12 can be formed.

A photosensitizing dye 14 is adsorbed on the semiconductor layer 13 thuslaminated. The photosensitizing dye 14 uses a ruthenium-based metalcomplex such ascis-bis(isothiocyanate)bis(2,2′-bipyridyl-4,4′-dicarboxylic acid)Ru(II).

In order to allow the photosensitizing dye 14 to be adsorbed on thesemiconductor layer 13, for example, in a case of the above-describedruthenium complex, the semiconductor layer 13 is dipped in a 3.0×10⁻⁴mol/L ethanol solution of the ruthenium complex for 20 hours and, then,ethanol is evaporated.

As for a counter electrode 16, a metal such as platinum or gold ispreferred. The counter electrode 16 is produced on a substrate 17 byusing a vacuum deposition method. The semiconductor layer 13 and thecounter electrode 16 are arranged opposite to each other and a spacebetween both electrodes is filled with an electrolyte layer 15.

As for the electrolyte layer 15, an electrolyte solution, or anelectrolyte in a gel state or a solid state can be used. As for theelectrolyte solution, a solution containing an oxidation-reductionsystem (redox pair) such as I⁻/I₂ is mentioned. Particularly, aglutaronitrile solution containing 0.6 mol/L of tetrapropyl ammoniumiodide[N(C₃H₇)₄] I and 5×10⁻² mol/L of iodine I₂ is used.

A side face of the cell is hermetically sealed by a sealing material 18such as an epoxy-based thermosetting resin, an acrylic-basedultraviolet-ray curing resin or water glass. In such a manner asdescribed above, the laminated semiconductor layer as shown in FIG. 1Bcan be incorporated into the dye-sensitized solar cell as shown in FIG.1A.

EMBODIMENT 2 A Case in Which Semiconductor Superfine Grains HavingDifferent Band Structures From One Another are Laminated

FIG. 3 is an enlarged cross-sectional view of a major portion of anegative electrode according to Embodiment 2 of the invention. Aschematic cross-sectional view of an entire dye-sensitized photochemicalcell is same as in Embodiment 1 as shown in FIG. 1A and, accordingly,omitted.

In Embodiment 1, a case in which thin films of the semiconductormaterials are laminated one on the other is shown, but, in Embodiment 2,a process comprising coating of a dispersion in a paste state containingsuperfine grains of the semiconductor material, evaporating of adispersion medium and sintering of the superfine grains is repeatedlyperformed on a plurality of semiconductor materials, to thereby producea laminate structure.

In a same manner as in Embodiment 1, by laminating a plurality ofsemiconductor materials having different band structures from oneanother on the transparent electrode (negative electrode) 12 in theorder of from the semiconductor material having a low LUMO to thathaving a high LUMO, a structure of an entire semiconductor layer 13 inwhich the energy level of the passage through which a conduction bandelectron is transferred is decreased in the direction of the transparentelectrode (negative electrode) 12 can be formed.

As for a transparent substrate 21, a transparent substrate having such aproperty and shape as allow light to be easily transmitted, for example,a transparent glass sheet or a transparent substrate of plastic ispreferably used. However, since in Embodiment 2 the manufacturingprocess comprises a step of sintering the semiconductor superfine grainsat about 500° C., it is practical to use the glass sheet. Since it isnot necessary that a substrate 27 is transmittable to light, an opaqueglass sheet, a plastic sheet, ceramic sheet or metal sheet may be used.

On a surface of the transparent substrate 21, a transparent electrode 22comprising tin oxide doped with antimony or fluorine, indium oxide dopedwith tin or the like is formed. The transparent electrode 22 is formedby any one of a sputtering method, a CVD (Chemical Vapor Deposition)method, a sol-gel method, a vacuum deposition method, a coating methodand the like.

A semiconductor layer 23 is constituted by a plurality of semiconductorlayers having different constitutional elements from one another. Take acase of FIG. 3 as an example, the semiconductor layer 23 is constitutedby semiconductor porous films 23A to 23D comprising 4 types ofsemiconductor materials A to D. Thickness of each layer is in the rangeof from approximately 10 nm to approximately 10 μm.

Materials applicable to constitutional materials of the semiconductorporous films 23A to 23D described in the order of from such materialhaving a high minimum energy level (LUMO) of the conduction band to thathaving a low minimum energy level (LUMO) of the conduction band are asfollows:

GaP, ZrO₂, Si, CdS, KTaO₃, CdSe, SrTiO₃, TiO₂, Nb₂O₅, ZnO, Fe₂O₃, WO₃,SnO₂ and In₂O₃.

Energy levels of LUMO and HOMO of various types of materials are thoseas described in Embodiment 1 and, also, in FIG. 2.

A plurality of semiconductor materials, that is, four types ofsemiconductor materials A to D in a case of FIG. 3, are selected fromamong the aforementioned semiconductor materials and, then, thin filmsof those materials are laminated on the transparent electrode 22 in theorder of from the thin film having a low LUMO of the conduction band tothat having a high LUMO of the conduction band.

Particularly, firstly, superfine grains of the semiconductor materialhaving the lowest minimum energy level of the conduction band aredispersed in an aqueous nitric acid solution or hydrochloric acid or thelike to form a dispersion in a paste state and, then, the thus-formeddispersion in the paste state is applied on the transparent electrode 22by a doctor blade method or the like and, after water is evaporatedtherefrom, sintered at about 500° C., to thereby produce thesemiconductor porous film 23A.

Next, the dispersion in the paste state which is prepared by dispersingsuperfine grains of the semiconductor material having the second lowestminimum energy level of LUMO is applied onto the semiconductor porousfilm 23A and then, after water is evaporated therefrom, sintered atabout 500 C, whereby the semiconductor porous film 23A is formed.

Further, the above-described process is repeated two more times to formthe semiconductor layer 23 which is consisted by the semiconductorporous film 23A to 23D which are laminated on the transparent electrode(negative electrode) 22 in order of from the semiconductor materialhaving a low minimum energy level of the conduction band to that havinga high minimum energy level.

In such a manner as described above, a structure of an entiresemiconductor layer 23 in which the energy level of the passage throughwhich a conduction band electron is transferred is decreased in thedirection of the transparent electrode (negative electrode) 22 can beformed.

A photosensitizing dye 24 is adsorbed on the thus-laminatedsemiconductor layer 23 comprising porous films. The photosensitizing dye24 uses a ruthenium-based metal complex such ascis-bis(isothiocyanate)bis(2,2′-bipyridyl-4,4′-dicarboxylic acid)Ru(II).

In order to allow the photosensitizing dye 24 to be adsorbed on thesemiconductor layer 23, for example, in a case of the above-describedruthenium complex, the semiconductor layer 23 is dipped in a 3.0×10⁻⁴mol/L ethanol solution of the ruthenium complex for 20 hours and, then,ethanol is evaporated.

As shown in FIG. 3, in a case of Embodiment 2, since thephotosensitizing dye 24 is adsorbed by being taken in the semiconductorfilm 23, the photosensitizing dye 24 comes to be in direct contact withthe semiconductor porous films 23A to 23D having different bandstructures from one another.

However, the photosensitizing dye 24 maybe adsorbed only on the surfaceof the semiconductor layer 23, instead of being taken therein.

In a case of the porous film formed from superfine grains of thesemiconductor material, the superfine grains are in point contact withone another and electric contacts between superfine grains are broken byspaces existing all around. Therefore, in comparison with a case ofusing a bulk layer having no space, a transfer path of the electroninjected from the photosensitizing dye 24 becomes complicated and,accordingly, it is considered to particularly effectively contribute tothe transfer of the electron which is the charge carrier that thesemiconductor layer 23 has the structure in which the energy level ofthe passage through which the conduction band electron is transferred isreduced in the direction of the transparent electrode (negativeelectrode) 22.

As for a counter electrode, a metal such as platinum or gold ispreferred. The counter electrode is produced on a substrate by using avacuum deposition method or the like. The semiconductor layer 23 and thecounter electrode are arranged opposite to each other and a spacebetween both electrodes is filled with an electrolyte layer.

As for the electrolyte layer, an electrolyte solution, or an electrolytein a gel state or a solid state can be used. As for the electrolytesolution, a solution containing an oxidation-reduction system (redoxpair) such as I⁻/I₂ is mentioned. Particularly, a glutaronitrilesolution containing 0.6 mol/L of tetrapropyl ammonium iodide and 5×10⁻²mol/L of iodine is used.

A side face of the cell is hermetically sealed by a sealing materialsuch as an epoxy-based thermosetting resin, an acrylic-basedultraviolet-ray curing resin or water glass. In such a manner asdescribed above, it becomes possible to incorporate the semiconductorlayer 23 comprising the laminated semiconductor porous films 23A to 23Das shown in FIG. 3 into the dye-sensitized solar cell as shown in FIG.1A.

EMBODIMENT 3 A Semiconductor Layer (1) Comprising a Complex of aTitanium Oxide Thin Film Having a Drift Region of an Electron Inside anda Titanium Oxide Fine Grain

Hereinafter, with reference to FIGS. 4A to 4G, a manufacturing processof a semiconductor layer 33 comprising a complex of a titanium thin filmhaving a drift region of an electron inside and a titanium oxide finegrain will be described.

Step 1

A thin film electrode 32 is formed on a surface of a substrate 31. Asfor the substrate 31, for example, a glass sheet is used. The thin filmelectrode 32 is, for example, a thin film of indium oxide doped with tin(ITO) or gold and is formed by a vapor deposition or sputtering.

Step 2

A TiO₂ thin film 41 is formed on the substrate 31. Such film forming maybe performed by sputtering or, otherwise, may be performed by a sol-gelmethod.

When the substrate 31 is titanium Ti, a TiO₂ layer 41 may be formed byusing an anodic oxidation coating.

Step 3

An impurity (dopant) is doped in the TiO₂ thin film 41 by using an ionimplantation method. The impurity effectively changes a forbidden bandwidth of the TiO₂ layer 41 and can use, for example, Cr, V, N, B or Al.

An ion implantation is performed from a surface 42 of the TiO₂ thin film41 and the impurity is doped in an innermost portion of the TiO₂ thinfilm 41. Succeedingly after the ion implantation, an activationannealing is performed. It is necessary to appropriately set anannealing condition depending on combinations of types of the impuritiesand ion implantation conditions and, for example, in a case of Cr, atemperature of 450° C. and thereabout can be used. At that time, animpurity diffusion layer 43 is formed by a thermal diffusion of theimpurity.

In the thus-formed impurity diffusion layer 43, there is an impurityconcentration distribution in which an impurity concentration in theinnermost portion thereof is highest and the impurity concentrationbecomes lower in the direction of the surface thereof. For this account,the energy level (LUMO) of the passage through which the conduction bandelectron is transferred becomes lower as a position moves from thesurface 42 to the thin film electrode 32 and, then, a structure in whichthe electron is accelerated in the direction of the thin film electrode32 (in the direction of thickness of the TiO₂ thin film 41) is formed.

At the time of performing the ion implantation, the aforementionedimpurity concentration distribution can also be formed by performing theion implantation with a high dosage by using a large accelerationvoltage and the ion implantation with a low dosage by using a smallacceleration voltage and, then, performing activation annealing.

In order to dope the impurity by the thermal diffusion, an impurity gas(dopant-containing gas) is introduced into an atmospheric gas, while theTiO₂ thin film 41 is being formed on the thin film electrode 32 by thesputtering method. At that time, an impurity gas concentration in theatmospheric gas is allowed to be high at an early time of the sputteringand, then, to be gradually reduced.

Step 4

After a resist film 44 is applied on the TiO₂ thin film 41, a pattern,for example, in a stripe state is formed on the resist film 44 by usinga photolithography technique.

Step 5

An etching is performed on the TiO₂ thin film 41 having the-thus formedpattern on the surface 42 thereof by ion milling or solution etching, tothereby form a multitude of grooves (concave portions) 45 on the surface42. Depth of such groove 45 is set to be in the range, for example, offrom about 1 μm to about 10 μm. Width of the groove 45 is set to be suchan extent as a sufficient amount of the paste of the TiO₂ fine grainscan enter in the subsequent step 6.

Step 6

Lastly, on the surface 42 of the TiO₂ thin film 41 having a groove 45thereon, a dispersion of the TiO₂ fine grains 47 in the paste state isapplied and, after a dispersion medium is evaporated, is sintered at atemperature of, preferably, from 450° C. to 550° C. and, morepreferably, 500° C. At that time, electric contacts between the TiO₂fine grains 47 and the surface of the TiO₂ thin film 41 on which aconcave portion 45 and a convex portion 46 are formed by being providedwith the groove 45 are formed, to thereby form a complex layer 33 of theTiO₂ thin film and the TiO₂ fine grains.

Step 7

Lastly, a photosensitizing dye is adsorbed on the TiO₂ fine grains 47.The photosensitizing dye which is a dye having an absorption band offrom 200 nm to 1500 nm uses a ruthenium-based metal complex, such ascis-bis(isothiocyanate)-bis(2,2′-bipyridyl-4,4′-dicarboxylicacid)Ru(II), or the like.

In a case of the aforementioned ruthenium complex, the TiO₂ fine grains47 are dipped ina 3.0×10⁻⁴ mol/L ethanol solution of ruthenium complexfor 20 hours and, then, ethanol is evaporated therefrom, to therebyallow the photosensitizing dye to be adsorbed on the TiO₂ fine grains47.

In the semiconductor layer 33 comprising the complex 33 of the titaniumoxide thin film 41 and the titanium oxide fine particles 47 as shown inFIG. 4G, as described before, a structure in which the conductionelectron is accelerated in the direction (in the direction of depth ofthe TiO₂ thin film 41) 49 of drawing the electron out as shown by anarrow in a dotted line is formed in the convex portion 46 of the thinfilm 41.

For this account, immediately after the conduction band electron (shownby the arrow) which is subjected to a charge separation from thephotosensitizing dye which absorbed light at the interface between thephotosensitive dye and the titanium oxide fine grains and drawn into theinside of the titanium oxide fine grains 47 reaches the convex portion46, the electron is subjected to a force in the direction 49 of drawingthe electron out and a movement of the conduction band electron iscontrolled such that it is drifted in the direction of the thin filmelectrode 32. As a result, the amount of the electrons to be drawnoutside, namely, an output current is increased.

Further, in comparison with the fine grain layer 47 in which spaces arelarge in number and fine grains are subjected to point contact with oneanother, since the convex portion 46 is a bulk layer having no space, ithas a large effective cross-sectional area and has a small resistance.Therefore, the energy of the conduction band electron which is lost bybeing converted into heat by means of an inner resistance becomessmaller in a case of transfer of the electron in the convex portion 46than in a case of diffusion thereof in the fine grain layer 47 by a samedistance and, then, an output voltage is increased.

On the other hand, since the photosensitizing dye is adsorbed on thesurface of the TiO₂ fine grain 47, an adsorption area is substantiallylarge compared with a case of being adsorbed on the TiO₂ thin film 41.Therefore, a larger amount of the photosensitizing dye can be adsorbedand, then, an amount of light to be absorbed can be increased.

As described above, a dye-sensitized photoelectric conversion apparatuscomprising the semiconductor layer 33 comprising the complex of the TiO₂thin film 41 and the TiO₂ fine grain 47 as shown in FIG. 4Gsimultaneously has two advantages of a low resistance of the bulk layerand a large surface area of the fine grain layer and, since a structureof leading the conduction band electron in the direction of drawing theelectron out is formed in the TiO₂ thin film 41, the output current andthe output voltage are simultaneously improved, to thereby enhance aneffective photoelectric conversion efficiency.

Further, as shown in FIG. 4G, when width of the groove (concave portion)45 is expressed by w, width of the convex portion by W and depth of theirregularity by d, w, W and d are preferably under such conditions asdescribed below.

As the groove 45 cut in the TiO₂ thin film 41 is formed more densely andthe depth of the groove (concave portion ) 45 is deeper, a contact areabetween the TiO₂ fine grain 47 and the TiO₂ thin film 41 becomes largerand a change of taking the conduction band electron inside the TiO₂ finegrains 47 in the drift region becomes larger. Also from such point asdescribed above, w and W are preferably as small as possible while d ispreferably as large as possible. However, when w is unduly small and dis unduly large, it becomes difficult to introduce the paste of the TiO₂fine grain 47 into the groove (concave portion) 45. Under thesecircumstances, the depth d of the groove 45 is set in the range, forexample, of from 1 μm to 10 μm approximately and the following relationsmay be established:1≦w/d≦2; and1≦w/W.

As for representative values, W=w=10 μm and d=5 μm.

EMBODIMENT 4 A Semiconductor Layer (2) Comprising a Complex of aTitanium Oxide Thin Film Having a Drift Region of an Electron Inside anda Titanium Oxide Fine Grain

FIGS. 5A and 5B are a schematic cross-sectional diagram showing asemiconductor layer 33 comprising a complex of a titanium oxide thinfilm and a titanium oxide fine grain according to Embodiment 4 and anenlarged cross-sectional view of a major portion thereof.

It is not necessary for the TiO₂ fine grain layer 47 as described inEmbodiment 3 to fill the groove (concave portion) 45 cut in the TiO₂thin film 41 out. As shown in FIGS. 5A and 5B, an irregular patternhaving the groove (concave portion) 45 wider than the thickness of theTiO₂ fine grain layer 47 may be provided on the surface 42 of the TiO₂thin film 41.

On this occasion, since the electrolyte layer 5 can be moved in a gap 50remaining in the groove (concave portion) 45, when the photosensitizingdye 4 adsorbed on the TiO₂ fine grain 48 of the concave portion 45 losesthe electron by absorbing light, the reducing agent (for example, I⁻) inthe electrolyte layer 5 becomes easy to advance to the photosensitizingdye 4. As a result, since the photosensitizing dye 4 which has lost theelectron is quickly reduced to be regenerated, even when light having ahigh light intensity is incident, it hardly occurs that thedye-sensitized photochemical cell is saturated.

In Embodiment 4, since other points than that described above are sameas those in Embodiment 3, it goes without saying that effects asdescribed in Embodiment 3 also exist in Embodiment 4.

The invention has so far been described with reference to embodiments;however, the invention is by no means limited thereto and it should beappreciated that various changes and modification can appropriately bemade without departing from the scope and spirit of the invention.

INDUSTRIAL APPLICATION

According to the present invention, since a region in which an energylevel of a passage through which a charge carrier is transferred isreduced in the direction of drawing the charge carrier out is formed inat least one portion inside a semiconductor layer which is a passage ofa charge carrier from an interface between a semiconductor layer and aphotosensitizing dye to a charge carrier drawing-out electrode, thecharge carrier is subjected to a force in the direction of drawing thecarrier out in this region and movement of the charge carrier isdirected in the direction of drawing the carrier out.

Namely, since transportation of the charge carrier after being implantedin the semiconductor layer is controlled based on the energy level ofthe passage through which the charge carrier is transferred in thesemiconductor layer, the number of charge carriers which can reach thecharge carrier drawing-out electrode is increased, to thereby enhanceenergy conversion efficiency.

1. A dye-sensitized photoelectric conversion apparatus, comprising: asemiconductor layer comprising a photosensitizing dye, wherein a chargecarrier generated by allowing light to be incident in thephotosensitizing dye can be drawn out through the semiconductor layer,wherein the semiconductor layer: is formed by a plurality of regions,having different energy levels from one another, of a passage throughwhich the charge carrier is transferred, and comprises the regions inwhich the energy levels in the semiconductor layer are reduced stepwiseand/or continuously in a direction of drawing the charge carrier out. 2.The dye-sensitized photoelectric conversion apparatus as set forth inclaim 1, wherein the semiconductor layer is formed by a plurality oflayers, having different minimum energy levels from one another, ofconduction band and the energy levels are reduced stepwise and/orcontinuously in the direction of drawing the charge carrier out.
 3. Thedye-sensitized photoelectric conversion apparatus as set forth in claim1, wherein the plurality of regions comprise a plurality ofsemiconductor materials in which constitutional elements are differentfrom one another.
 4. The dye-sensitized photoelectric conversionapparatus as set forth in claim 1, wherein the plurality of regionscomprise semiconductor materials comprising same constitutional elementswith one another and ratios of the constitutional elements are changedstepwise and/or continuously in the direction of drawing the chargecarrier out.
 5. The dye-sensitized photoelectric conversion apparatus asset forth in claim 1, wherein the plurality of regions comprisesemiconductor materials which are of a same element composition and areof different dopants from one another.
 6. The dye-sensitizedphotoelectric conversion apparatus as set forth in claim 1, wherein theplurality of regions comprise materials in which a same dopant is dopedin a semiconductor material having a same element composition and aconcentration of the dopant is changed stepwise and/or continuously inthe direction of drawing the charge carrier out.
 7. The dye-sensitizedphotoelectric conversion apparatus as set forth in claim 1, wherein thephotosensitizing dye is adhered on a surface of the semiconductor layeror impregnated inside the semiconductor layer.
 8. The dye-sensitizedphotoelectric conversion apparatus as set forth in claim 1, wherein, ona side to which the photosensitizing dye is adhered, an irregularcontour is formed on a first semiconductor layer and, by joining theirregular contour with a second semiconductor layer, the semiconductorlayer is comprised.
 9. The dye-sensitized photoelectric conversionapparatus as set forth in claim 1, wherein the semiconductor layercomprising the photosensitizing dye and an electrolyte layer arelaminated together between a pair of electrodes.
 10. The dye-sensitizedphotoelectric conversion apparatus as set forth in claim 1, beingcomprised as a dye-sensitized photochemical cell.
 11. A method forproducing a dye-sensitized photoelectric conversion apparatus whichcomprises a semiconductor layer comprising a photosensitizing dye andwherein a charge carrier generated by allowing light to be incident inthe photosensitizing dye can be drawn out through the semiconductorlayer, the method comprising the steps of: constituting thesemiconductor layer by a plurality of regions, having different energylevels from one another, of a passage through which the charge carrieris transferred; and arranging the plurality of regions such that theenergy levels are reduced stepwise and/or continuously in a direction ofdrawing the charge carrier out.
 12. The method for producing adye-sensitized photoelectric conversion apparatus as set forth in claim11, wherein the semiconductor layer is constituted by a plurality oflayers, having different minimum energy levels from one another, ofconduction band and the energy levels are reduced stepwise and/orcontinuously in the direction of drawing the charge carrier out.
 13. Themethod for producing a dye-sensitized photoelectric conversion apparatusas set forth in claim 11, wherein a plurality of semiconductor materialshaving different constitutional elements from one another are laminatedtogether.
 14. The method for producing a dye-sensitized photoelectricconversion apparatus as set forth in claim 13, wherein a thin filmcomprising the semiconductor material is formed by a sputtering methodor a sol-gel method.
 15. The method for producing a dye-sensitizedphotoelectric conversion apparatus as set forth in claim 13, wherein astep comprising coating of a dispersion containing a superfine grain ofa semiconductor material, evaporating of a dispersion medium, andsintering, melt-fusing or bonding of the superfine grain is repeatedlyperformed on the plurality of semiconductor materials.
 16. The methodfor producing a dye-sensitized photoelectric conversion apparatus as setforth in claim 11, wherein, when a step of doping a plurality of typesof dopants in the semiconductor layer by means of an ion implantation isperformed, a dopant having a large effect in reducing the energy levelis doped in an inner portion by using a large acceleration voltage bymeans of the ion implantation.
 17. The method for producing adye-sensitized photoelectric conversion apparatus as set forth in claim11, wherein, when a step of doping a single type of dopant in thesemiconductor layer by means of an ion implantation is performed, theion implantation by a large acceleration voltage is performed with ahigh dosage while the ion implantation by a small acceleration voltageis performed with a low dosage.
 18. The method for producing adye-sensitized photoelectric conversion apparatus as set forth in claim11, wherein, when a step of doping a single type of dopant in thesemiconductor layer by means of an ion implantation is performed, adopant concentration distribution is changed by a thermal diffusionafter a dopant implantation.
 19. The method for producing adye-sensitized photoelectric conversion apparatus as set forth in claim11, wherein the energy levels are changed by implanting an oxygen ion inthe semiconductor layer.
 20. The method for producing a dye-sensitizedphotoelectric conversion apparatus as set forth in claim 11, wherein,when a step is performed of doping a plurality of types of dopants byintroducing a dopant-containing gas into an atmospheric gas while thesemiconductor layer is being formed by a sputtering method, such dopingis performed in the order of from a dopant having a large effect inreducing the energy levels to a dopant having a small effect in reducingthe energy levels.
 21. The method for producing a dye-sensitizedphotoelectric conversion apparatus as set forth in claim 11, wherein,when a step of doping a single type of dopant by introducing adopant-containing gas into an atmospheric gas while the semiconductorlayer is being formed by a sputtering method is performed, aconcentration of the dopant-containing gas is reduced.
 22. The methodfor producing a dye-sensitized photoelectric conversion apparatus as setforth in claim 11, wherein the photosensitizing dye is adhered on asurface of the semiconductor layer or impregnated inside thesemiconductor layer.
 23. The method for producing a dye-sensitizedphotoelectric conversion apparatus as set forth in claim 11, wherein, ona side to which the photosensitizing dye is adhered, an irregularcontour is formed on a first semiconductor layer and, by joining theirregular contour with a second semiconductor layer, the semiconductorlayer is formed.
 24. The dye-sensitized photoelectric conversion method,wherein the semiconductor layer comprising the photosensitizing dye andan electrolyte layer are laminated together between a pair ofelectrodes.